1. Field of the Invention
The present invention is directed to a sulfur, selenium, and/or tellurium lamps having improved color rendering.
2. Related Art
The invention relates to sulfur selenium and/or tellurium lamps (hereinafter the xe2x80x9csubject lampsxe2x80x9d), such as those described in U.S. Pat. Nos. 5,404,076, 5,661,365, and 5,688,064, each of which is herein incorporated by reference in its entirety. In the subject lamps, elemental sulfur, selenium and/or tellurium is present in gaseous form, which is obtainable when the fill is excited by sufficient power, in an amount such that the excited fill emits a discharge of visible radiation from the fill component with substantially all of such radiation being molecular radiation which is emitted in the visible region of the spectrum. The subject lamps disclosed in the above-mentioned patents are discharge lamps, and may be either of the electrodeless type where the discharge is excited by microwave or RF power, or of the electroded type where the discharge is excited by an electrical voltage across the electrodes.
The subject lamps are highly efficient for visible lighting with good color rendering. By way of comparison, the color rendering index (CRI) for a sulfur lamp is about 80, as opposed to a CRI of about 70 for the metal halides lamps, a CRI of about 62 for fluorescent lamps, and a CRI of about 22 for high pressure sodium lamps. A lamp with a CRI equal to or higher than about 90 would be considered a high quality color rendering lamp.
The addition of metal halides to HID lamps is a common practice in the lighting industry. For most metal halide additives, the metal atoms are excited, ionized and then radiate at the desired spectral region. This visible radiation from excited atoms is typically accompanied by unwanted infrared line radiation which leads to lower efficacy. Optimizing the color of metal halide lamps is accomplished by changing the ratio of other metal halides to provide sufficient amounts of the blue and green radiation. For example, U.S. Pat. Nos. 3,852,630, 4,360,758, 4,742,268, 4,027,190, and 4,801,846 disclose that calcium, strontium and aluminum halides can be used with mercury containing metal halide lamps to bring up the red output to increase the CRI.
If a CRI of about 90 could be achieved for the subject lamps without substantially lowering the efficacy, excellent lamps for a wider variety of lighting applications would result. The subject lamps produce visible light efficiently through self absorption of ultraviolet radiation in an optically thick plasma. Any attempt to increase the CRI, however, is limited by the full width of half maximum (FWHM) of the visible spectrum of the lamps. In other words, an increase in red radiation results in a loss of blue radiation, thereby lowering the CRI. Blue or green radiation cannot be substantially increased by introducing metal halides into sulfur plasma, because sulfur molecules have strong self absorption in those regions.
In accordance with a first aspect of the present invention, calcium and/or strontium halide is added to the fill of a sulfur, selenium, and/or tellurium lamp to improve the color rendering index. Optionally, an inert starting gas such as argon, xenon, or krypton is also included in the fill.
In accordance with a further aspect of the invention a metal halide volatilizer is also added to the fill to increase the vapor pressure of the calcium or strontium halide.
In accordance with another aspect of the invention, a sulfur lamp with a calcium halide additive surprisingly maintains a high CRI after over several thousands hours of lamp operation.
In one embodiment of the invention, a discharge lamp bulb for providing visible radiation includes a lamp envelope which is made of light transmissive material. A fill in the envelope includes at least one first member selected from the group consisting of calcium halide and strontium halide, and at least one second member selected from the group consisting of elemental sulfur and elemental selenium in gaseous form which is obtainable when the fill is excited by sufficient power in operation, in an amount such that the excited fill emits a discharge of visible radiation from the selected members with substantially all of the radiation being molecular radiation which is emitted in the visible region of the spectrum. The calcium halide may be one of CaBr2, Cal2, and CaCl2, and the strontium halide may be one of SrBr2, Srl2, and SrCl2.
Preferably, the concentration of the sulfur, if present in the fill, is between about 0.1 mg/cc and 5 mg/cc, the concentration of the selenium, if present in the fill, is between about 0.05 mg/cc and 2 mg/cc, and the concentration of each of CaBr2 and SrBr2 is between about 0.001 mg/cc and 1 mg/cc. The fill may also include a metal halide volatilizer including at least one of aluminum halide, gallium halide, germanium halide, indium halide, tin halide, and iron halide, or compounds thereof. If so, the concentration of each metal halide volatilizer compound is between about 0.01 mg/cc and 2 mg/cc.
The lamp may be electrodeless or electroded and is preferably used in combination with means for exciting a discharge in the fill, which may include, for example, means for generating microwave or RF power and means for coupling the microwave or RF power to the fill.